In WDM (wavelength division multiplex) optical communication, multiple component wavelengths of light each carry a communication signal. Each of the multiple component wavelengths of light form a WDM channel. An OADM (optical add-drop multiplexer) is used for WDM signal management. WDM signals are transmitted from location to location using the channels. At a particular location, the signal within each channel is either passed for transmission to another location, or is dropped for local distribution. As signals are dropped, the channels corresponding to those dropped signals are free to accept new signals. The new signals are uploaded into the WDM signal at the same wavelength as the signal that was dropped. Maintaining an active signal in each channel maximizes total bandwidth. Optical devices are often used to provide the switching within an OADM. Exemplary optical devices, and methods for making the same, are disclosed in U.S. Pat. Nos. 5,311,360, 5,841,579 and 5,808,797, issued to Bloom et al., and U.S. Pat. No. 6,268,952 issued to Godil et al., the contents of which are hereby incorporated by reference.
Dynamic gain equalization is also an aspect of WDM signal management. A variety of dynamic equalization techniques have been advanced, which seek to equalize component signals in a WDM system. Most rely on some spectral multiplexer/demultiplexer component, followed by an electrically-controllable variable optical attenuator which can operate on the de-multiplexed channels (or possibly a band of channels). Component signal intensity exiting the dynamic gain equalizer is set according to desired performance parameters, which may or may not require that all wavelengths have the same power. Light modulators are often used as the variable optical attenuator within a dynamic gain equalizer. Exemplary dynamic gain equalizers including optical devices are disclosed in U.S. application Ser. No. 10/051,972, filed on Jan. 15, 2002, and entitled “Method and Apparatus for Dynamic Equalization in Wavelength Division Multiplexing”, the contents of which are hereby incorporated by reference.
Many applications require the equalization of the output spectrum as well as excellent extinction in the non-lit fibers. For example, switching input light from one channel to another can be achieved by diffracting the light into a first order of light, while reflecting very little light, ideally no light, as specularly reflected zero order light. The diffracted first order light in this case is then attenuated by controlled means, thereby equalizing the light that has been “switched” into the first order. It is common practice to perform the switching and equalizing functions at the same physical location for convenience, maintenance, and economic advantages. Switching and equalization together is performed by a wavelength selective switch and equalizer (WSSE).
FIG. 1 illustrates an exemplary operational schematic of a conventional 1×2 WSSE 5. The input signal IN comprises three component wavelength signals λ1, λ2 and λ3. In this case, the component wavelength signal 12 is switched to OUT2, the component wavelength signals λ1 and 13 are switched to OUT1 and the component wavelength signals λ1 and λ3 are equalized to the same level as component wavelength signal λ2.
FIG. 2 illustrates a functional schematic of the 1×2 WSSE 5 illustrated in FIG. 1. The functional schematic of FIG. 2 illustrates the steps required to perform the operation illustrated in FIG. 1. To perform the operation illustrated in FIG. 1, two steps are required. First, the component wavelength signals λ1 and 13 are switched and equalized by a 1×2 WSSE 10 to Intermediate 1. However, to equalize the component wavelength signals λ1 and λ3, attenuated portions of the component wavelength signals λ1 and λ3 are directed to Intermediate 2. Therefore, it is then necessary to equalize Intermediate 2 to remove the attenuated portions of the component wavelength signals 11 and 13. Second, Intermediate 2 is equalized by a 1×1 wavelength selective equalizer (WSE) to eliminate the attenuated portions of the component wavelength signals λ1 and λ3. This results in the equalized component wavelength signal λ2 at OUT2.
In this case, the Intermediate 1 comprises the intended output of equalized component wavelength signals λ1 and λ3. Therefore, in this case, a 1×1 WSE 15 merely passes through Intermediate 1 as OUT1. However, it should be clear that 1×1 WSE 15 is necessary in the case where component wavelength signal λ2 is to be switched to OUT2 and equalized. This is due to when the component wavelength signal λ2 is equalized by the 1×2 WSSE 10, an attenuated portion of the component wavelength signal λ2 is directed to Intermediate 1. Intermediate 1 is then equalized by 1×1 WSE 15 to eliminate the attenuated portion of the component wavelength signal λ2. The 1×1 WSE 15 and 20 each include a light modulator to equalize the intermediate signals, Intermediate 1 and 2. It is understood that although the WSSE described in relation to FIGS. 1 and 2 relates to a 1×2 WSSE, the same process and functionality readily applies to a 1×N WSSE.
It is understood that other means for equalizing the intermediate signals are possible. Regardless of the nature of the other means for equalizing, it is inefficient to use the 1×2 WSSE 5 and the other means for equalizing to perform the switch and equalize functions.
What is needed is a wavelength signal switch and equalizer that is more efficient than the conventional two-step process. What is further needed is a more efficient wavelength selective switch and equalizer that is more easily produced, and produced at a reduced cost.